IMHO, object oriented programming is a concept that exists at a higher level of abstraction than procedural programming. The two are not mutually exclusive in that individual methods in an OO program look pretty much the same as individual functions in a procedural program. This contrasts with, for example, functional programming, which requires a completely different mindset. Furthermore, you can write procedurally in an OO language by making everything static, etc. You can be a human compiler and write effectively OO code in C by using lots of function pointers and struct pointer casting.
OO, then, is more of a design philosophy and world view than something w/ a rigorous definition. It requires that inheritance, polymorphism, etc. be used as major patterns in structuring your code, and that syntax be provided to make these expressible without resorting to low-level tricks. It requires that you think of code that acts on the state of a collection of data as being a property of the data, not a procedure that exists by itself. It is not black and white. Your code can be "more" or "less" OO depending on how heavily you rely on inheritance, polymorphism, classes and the "methods as a property of data" worldview as a means of structuring and explaining/understanding your code.
